Generation of gravity waves from thermal tides in the Venus atmosphere

Abstract

Gravity waves play essential roles in the terrestrial atmosphere because they propagate far from source regions and transport momentum and energy globally. Gravity waves are also observed in the Venus atmosphere, but their characteristics have been poorly understood. Here we demonstrate activities of small-scale gravity waves using a high-resolution Venus general circulation model with less than 20 and 0.25 km in the horizontal and vertical grid intervals, respectively. We find spontaneous gravity wave radiation from nearly balanced flows. In the upper cloud layer (~70 km), the thermal tides in the super-rotation are primary sources of small-scale gravity waves in the low-latitudes. Baroclinic/barotropic waves are also essential sources in the mid- and high-latitudes. The small-scale gravity waves affect the three-dimensional structure of the super-rotation and contribute to material mixing through their breaking processes. They propagate vertically and transport momentum globally, which decelerates the super-rotation in the upper cloud layer (~70 km) and accelerates it above ~80 km.

Longitudinally Asymmetric Stratospheric Oscillation on a Tidally Locked Exoplanet

“…To adapt u′ to this asymmetrical background flow, we use a high-pass filter to remove the influence of waves with zonal wavenumber less than 5 (i.e., long Rossby waves), following Sugimoto et al (2021). The remaining waves are resolved longwavelength gravity and inertio-gravity waves.…”

“…To understand how a tidally locked orbital configuration affects where gravity waves are generated on the planet, we use the Lagrangian Rossby number, Ro ( L) , to identify resolved gravity wave sources. Ro ( L) is not an exclusive measure of gravity wave sources, but is often used to detect unbalanced flows that give rise to gravity waves (Zhang et al 2000;Lin 2007;Sugimoto et al 2021). It is defined as the ratio of the acceleration of a parcel of air to the Coriolis acceleration and essentially measures the local departure from geostrophy:…”

“…We modify Ro ( L) in several respects to adapt the metric to a slowly rotating planet. Following Sugimoto et al (2021), we disregard the local wind tendency term, ¶ ¶ v t h , and fix the value of f near the equator ( ±10°) equal to f at 10°to extend the metric to cover the equatorial region. As the slowly rotating Proxima Centauri b is not in geostrophic balance, the resulting values of Ro ( L) are very large.…”

Longitudinally asymmetric stratospheric oscillation on a tidally locked exoplanet

“…The planet's stationary Rossby waves skew the zonal anomaly so that it represents the Rossby waves rather than transient flows. To adapt u ′ to this asymmetrical background flow, we use a high-pass filter to remove the influence of waves with zonal wavenumber less than 5 (i.e., long Rossby waves), following Sugimoto et al (2021). The remaining waves are resolved long wavelength gravity and inertio-gravity waves.…”

“…To understand how a tidally locked orbital configuration affects where gravity waves are generated on the planet we use the Lagrangian Rossby number, Ro (L) , to identify resolved gravity wave sources. Ro (L) is not an exclusive measure of gravity wave sources, but is often used to detect unbalanced flows which give rise to gravity waves (Zhang et al 2000;Lin 2007;Sugimoto et al 2021). It is defined as the ratio of the acceleration of a parcel of air to the Coriolis acceleration and essentially measures the local departure from geostrophy:…”